This invention is a method and apparatus for measuring deformation objects and for nondestructive testing, and, in particular, includes a new method of practicing the technique of shearing interferometry, or "shearography".
Conventional holographic interferometry has been used for measuring surface displacements and can be used for detecting defects in test objects. A hologram is produced by illuminating an object with coherent light, generated by a laser, and causing the light reflected from the object to interfere with a "reference beam" coming directly from the laser. The interference pattern produced by the object beam and the reference beam constitutes the hologram. The pattern can be recorded on a photographic plate, or by other means. To test the object for defects, the object is stressed, and another hologram is taken on the same photographic plate. The result is a second-order interference pattern which yields information concerning the presence or absence of defects in the object.
It has long been recognized that holography is impractical for use as a tool for measurement of displacement and nondestructive testing in an industrial environment. Producing a hologram requires a relatively complex and sensitive optical apparatus; one needs a set of mirrors to direct the object beam and reference beam in the required directions. If the mirrors are not properly aligned, no hologram will be formed.
A hologram can also be ruined by environmental disturbances. To make a hologram, one often needs special equipment to isolate the optical elements and the object from stray vibrations. This need is particularly acute when the hologram is to be made in an industrial environment or in the field. Although these requirements may be relaxed when a pulsed laser is used to record the hologram, a pulsed laser is a very delicate instrument which is not only expensive but also requires highly-skilled personnel to operate and maintain.
Another disadvantage of holography is that it requires light having a relatively high degree of coherence. The "coherent length" requirement is comparatively high, in holography, because the total path length variation of a three-dimensional object is relatively large.
Holography also has the disadvantage that it is sensitive to rigid body motion. If a test object is tilted, the effect on the hologram will be different for different points on the object. This effect is due to the fact that each point on the hologram results from a combination of a beam reflected from the object and a fixed reference beam. Rigid body motion creates unwanted fringes in a hologram, as illustrated in FIG. 9 of the article by Y. Y. Hung, entitled "Shearography: A New Optical Method for Strain Measurement and Nondestructive Testing", Optical Engineering, May-June 1982, pages 391-5. These fringes can obscure or obliterate the useful information contained in the hologram. A comparison of shearography versus holography in nondestructive testing is given in the article of Y. Y. Hung, entitled "Shearography versus Holography in Nondestructive Evaluation", published in Proceedings of SPIE-The International Society for Optical Engineering, volume 604 (1986), at page 18.
For the reasons given above, holography has been generally limited to use in laboratories. It is normally impractical to use holography as a means of analyzing thousands of mass-produced parts as they are traveling along an assembly line.
An alternative to conventional holography, in the field of nondestructive testing, is shearing interferometry, or shearography. The latter terms are used interchangeably herein. In shearing interferometry, one produces a pair of laterally-displaced images of the test object, on the same image plane. The lateral displacement of images accounts for the term "shearing". The sheared images combine to form an interference pattern. When the light used is reasonably coherent, this interference pattern appears as a random interference pattern or set of "speckles", and the pattern is therefore often called a "speckle pattern". Comparison of the speckle patterns obtained while the object is deformed and while it is not deformed yields information about the integrity of the object.
Examples of patents showing means for practicing various forms of shearing interferometry include U.S. Pat. Nos. 4,139,302, 3,532,431, 4,118,124, 3,829,219, and 3,218,916. The disclosures of the latter patents are incorporated by reference herein.
U.S. Pat. No. 4,139,302 shows a method of shearography in which the sheared images are produced by an optical wedge, placed near a portion of an imaging lens. Light reflected from each point on the object passes through the lens alone, and also through the combination of the lens and the wedge. The wedge thus produces two laterally-displaced images at a detector, and these images interfere to form a speckle pattern.
The wedge device of the above-cited patent avoids some of the problems inherent in holography, such as the need for a matched object beam and reference beam, because it uses a single laser beam to produce a pair of sheared images. But use of the wedge introduces some new problems. By its nature, the wedge distorts the images; one of the images will necessarily be tilted somewhat, and will not be parallel to the image plane. The wedge system is also not conveniently adjustable; to vary the amount of shearing, one must replace the wedge with another wedge having a different geometry. There is thus no easy way of continuously adjusting the amount of shearing. Tilting the wedge is not a practical means of adjusting the shearing because the wedge needs to be in an optimum position to achieve best results.
Moreover, the amount of shearing in the wedge shearography system is limited by the wedge angle. To produce large amounts of shear, thus allowing two distantly separated regions of the same object or two different objects to be compared, a wedge having a large angle is needed. But a wedge of large angle will produce too much distortion in the images, and will yield test results of poor quality.
The present invention provides an improved method and apparatus for practicing shearography. The invention avoids the disadvantages inherent in holography, and also solves the problems, discussed above, encountered in the use of conventional shearographic methods.